Optical communication systems are known in which optical signals carrying data are transmitted from a transmit node to a receive node over an optical fiber. At the receive node, the optical signals are converted into corresponding electrical signals, which are then further processed.
Various techniques have been proposed to increase the data-carrying capacity of optical communication systems. For example, polarization multiplexing schemes have been implemented in which optical signals having different polarizations, but the same wavelength, are combined. Since each polarization can carry independent data streams, polarization multiplexing can have twice the capacity as a system transmitting light having a single polarization.
In another approach, referred to as quadrature amplitude modulation (QAM), the phase and amplitude of an optical signal may be modulated to carry a symbol, wherein the symbol includes multiple bits. For example, in accordance with a 4-QAM modulation format, the phase of an optical signal may be modulated to be in one of four states, each of which representing a corresponding one of four symbols. For example, each of phase angles of 45°, 135°, 225°, and 315° may correspond to the following bit combinations (symbols) 00, 01, 10, and 11, respectively.
Graphically, each state may be represented by a point on a complex plane and a collection of such points constitutes a constellation. The constellation associated with the 4-QAM modulation format has four points equally spaced from each other on the complex plane. Each point is associated with one of the four phase angles, and the distance each point is away from the origin of the complex plane corresponds to a magnitude of the corresponding state. Typically, in 4-QAM modulation, the magnitude associated with each phase angle (i.e., the distance each point is from the origin) is the same. 4-QAM may also be referred to as quadrature phase shift keying (QPSK).
In order to further increase capacity, higher order QAM modulation formats have been proposed. For example, an 8-QAM modulation format is associated with an eight point constellation, with each point of the constellation being associated with a particular combination of three bits. The 8-QAM constellation includes inner and outer sets of constellation points. The inner set includes four points that are spaced about and provided the same distance away from the origin and resembles the four points of the QPSK modulation format. The outer set also includes four points, but the points have a greater magnitude than that of the inner points, and are thus spaced farther away from the origin. Each of the outer four points is associated with a corresponding one of a plurality of phase angles, which are different from each other and the phase angles of the inner points.
Feedforward carrier recovery (FFCR) is a known technique for recovering the phase of a modulated optical signal. In particular, when the modulated signal is the modulated output of a laser having a large linewidth, such as certain distributed feedback (DFB) lasers, FFCR can provide optimum tracking of the signal. Feedback techniques are also known to provide carrier recovery.
Although FFCR and feedback techniques are effective in demoudlating QPSK modulated optical signals, for example, the carrier of a high order QAM modulated optical signal, such as 8-QAM, may be more difficult to recover because the outer points do not have the same phase angle as the inner points. According, feedforward and feedback carrier recovery technique are needed that can operate on 8-QAM and other higher order QAM modulation formats.